Silicon devices, both MOS and bipolar, emit light at sites of faulty device action. Consequently, the combination of detection of such light and the location of its emission is highly useful in device failure analysis. This is especially true for the complex very-large-scale integrated circuits, which often contain tens of thousands of transistors, making it difficult topinpoint the primary failure location by other means. We will report on a sensitive and sophisticated apparatus for accomplishing these &oals. Involved are an optical microscope, video cameras of high sensitivity for the emissions, and a powerful digital image processor, which greatly extends the ability to detect faint self-luminescing sites in the presence of video noise. The emitted light is the result of two distinct mechanisms. One mechanism is recombination of excess minority carriers in forward-biased junctions. This occurs often in a parasitic p-n-p-n structure common to CMOS circuits. If the p-n-p-n "latches", that is, large current flows irrespective of the device normal functions, two junctions are heavily forward biased. The emitted lisht is in a narrow band at 1.15 pm, and generally of very low intensity, as the probability of a radiative recombination is very small. The other mechanism is that of impact ionization, which results in broad-band emissions throughout the visible and into the near IR. This emission is characteristic, for example, of avalanche breakdown. But, it is of particular interest in state of the art short channel MOS devices, where hot carriers in the channel can result in instabilities of theg ate oxide.